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Environmental and Stress Physiology

Circumnutations of Sunflower Hypocotyls in Satellite Orbit ¹

Gravitational Plant Physiology Laboratory of the University City Science Center and Biology Department, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6017

The principal objective of the research reported here was to determine whether a plant's periodic growth oscillations, called circumnutations, would persist in the absence of a significant gravitational or inertial force. The definitive experiment was made possible by access to the condition of protracted near weightlessness in an earth satellite. The experiment, performed during the first flight of Spacelab on the National Aeronautics and Space Administration shuttle, Columbia, in November and December, 1983, tested a biophysical model, proposed in 1967, that might account for circumnutation as a gravity-dependent growth response. However, circumnutations were observed in microgravity. They continued for many hours without stimulation by a significant g-force. Therefore, neither a gravitational nor an inertial g-force was an absolute requirement for initation or continuation of circumnutation. On average, circumnutation was significantly more vigorous in satellite orbit than on earth-based clinostats. Therefore, at least for sunflower (Helianthus annuus L.) circumnutation, clinostatting is not the functional equivalent of weightlessness.

¹ Funding was provided by National Aeronautics and Space Administration grants and contracts NGR 39-030-010 and NAS 39-15340 to the University City Science Center and by NGR 39-010-149 and NAS 9-15531 to the University of Pennsylvania.

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